1. Field of the Invention
The invention relates to a nanotube array and to a method for producing a nanotube array.
As ongoing miniaturization continues, conventional silicon microelectronics will reach its limit. In particular, in the next ten years the development of ever smaller and more densely arranged transistors, which by now amount to several hundred millions of transistors per chip, will in principle encounter physical problems and limits. When feature sizes drop below 80 nm, the components will be disruptively affected by quantum effects, and these effects will become dominant at feature sizes of approximately 30 nm. The increasing integration density of the components on a chip also leads to a dramatic increase in the waste heat which is generated.
2. Description of the Related Prior Art
Carbon nanotubes are known to be a possible successor technology to conventional semiconductor electronics. By way of example, [1] gives an overview of this technology.
A nanotube is a single-walled or multiwalled, tube-like carbon compound. In the case of multiwalled nanotubes, at least one inner nanotube is coaxially surrounded by an outer nanotube. Single-walled nanotubes typically have diameters of approximately one nanometer, while the length of a nanotube may be several hundreds of nanometers. The ends of a nanotube are often closed off by means of in each case half a part of a fullerene molecule.
The extended π-electron system and the geometric structure of nanotubes result in good electrical conductivity, and consequently nanotubes are suitable for the construction of circuits with dimensions in the nanometer range. It is known from [2] that the electrical conductivity of carbon nanotubes may significantly exceed that of metals of the same dimensions.
The diameter and chirality of a nanotube are parameters on which the electrical conductivity of the nanotube is dependent. The electrical conductivity of a nanotube may furthermore be altered by applying an electric field and/or doping the nanotubes with boron nitride. In the latter case, it is customary to refer to a nanotube doped with boron atoms and nitrogen atoms or to a boron nitride nanotube.
On account of the conductivity of nanotubes and on account of the possibility of adjusting this conductivity in the manner described above, nanotubes are suitable for a wide range of applications, for example for the electrical connection technology in integrated circuits, for microelectronics components and for electron emitters.
Furthermore, it is known from [3] that the electrical resistance of nanotubes changes by approximately three orders of magnitude within a few seconds if nanotubes are exposed to a gas, such as for example a nitrogen dioxide (NO2) gas or an ammonia (NH3) gas. In an NH3 atmosphere, the electrical conductivity of the nanotubes is reduced, which can be explained by a shift in the valence band edge to well below the Fermi level of the nanotubes with a resultant charge carrier depletion. Conversely, the electrical conductivity of the nanotubes rises by approximately three orders of magnitude if the nanotubes are exposed to an NO2 atmosphere in a concentration of approximately 200 ppm. This can be explained by the fact that the Fermi energy of the nanotubes is shifted closer to the valence band and accordingly the number of charge carriers in the nanotubes increases.
For nanotubes to be used in microelectronics, it is often desirable for nanotubes to be applied in a defined manner at specific locations of a substrate. By way of example, nanotubes can be used as conductors in order to couple two conductor levels of a microcircuit element which are electrically separated from one another. For this purpose, it is necessary for nanotubes to be grown only where a corresponding electric coupling is desired, whereas the other regions of the substrate should remain clear of nanotubes in order to prevent electrical short circuits.
To achieve this objective, it is known to use a sputtering process to apply a metal which catalyzes the growth of nanotubes, for example iron, to a substrate which has been patterned, for example, with photoresist. Then, the patterned photoresist and the metal located thereon are removed using a lift-off method. As a result, the metal material remains only on locations on the substrate which were previously uncovered. The catalytically active metal which remains is used as a matrix for nanotubes to grow on.
Vapor deposition processes (chemical vapor deposition, CVD) are known processes for the production of carbon nanotubes. In the CVD process, the components and dopants are brought together as gases, if appropriate with additional carrier gases, in a reaction space, where the deposition on the substrate takes place. To produce carbon nanotubes using the CVD process, the carbon source used is often methane (CH4) or alternatively acetylene (C2H2).
Methods for producing nanotubes and nanowires on catalytically active surfaces using the CVD process have been described, for example, in [4] and [5]. The method described in those documents makes it possible to produce carbon nanotubes which are arranged vertically on a substrate.
However, in the method which is described in [4], the base has to consist of aluminum. This is a disadvantageous restriction in terms of the material. Furthermore, the method described in [4] results in relatively large, multiwalled carbon nanotubes with diameters of approximately 50 nm. Also, the carbon nanotubes produced using the method described are oriented perpendicular to the substrate and can therefore only be integrated in conventional silicon microelectronics to a limited extent.
The method for producing carbon nanotubes which is described in [5] results in the formation of an arrangement of carbon nanotubes in densely packed blocks. These blocks are defined by the catalyst (for example iron) which has been vaporized on by means of a mask. However, it is difficult to produce a regular arrangement using the production method described. Since once again it is only possible to produce nanotubes which are oriented perpendicular to the surface of the substrate, there are considerable limitations to the way in which the nanotubes can be coupled to conventional silicon microelectronics.
To summarize, methods for producing an array of carbon nanotubes which are known from the prior art have a number of drawbacks. For example, the nanotubes produced using the methods described are oriented perpendicular to the surface of the substrate. Furthermore, according to the known methods it is difficult to produce structurally defined arrays of nanotubes. There is no precisely defined direction of growth for the nanotubes on the surface of a catalyst material. The lack of order which results and the fact that the nanotubes are oriented perpendicular to the substrate surface means that the nanotube arrays which are known from the prior art cannot be coupled to conventional silicon microelectronics or can only do so with difficulty.
It is known from [6] to form catalyst islands on a substrate. A carbon nanotube which is coupled to two catalyst islands can be formed using a CVD process if a carbon nanotube which grows from a catalyst island happens randomly to grow toward a second catalyst island. Therefore, once again according to [6] it is not possible to produce a spatially sufficiently well defined array of nanotubes.